In the recent Dover trial, and elsewhere, the 'Intelligent Design' movement has championed the bacterial flagellum as an irreducibly complex system that, it is claimed, could not have evolved through natural selection. Here we explore the arguments in favour of viewing bacterial flagella as evolved, rather than designed, entities. We dismiss the need for any great conceptual leaps in creating a model of flagellar evolution and speculate as to how an experimental programme focused on this topic might look.
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We thank D. Blair, P. Aldridge and R. Berry for critical comments on this manuscript.
Nicholas J. Matzke is employed by the National Center for Science Education, a not-for-profit organization that defends the teaching of evolution in public schools.
Entrez Genome Project
- β-clip domain
A fold found in a diverse group of protein domains typified by the presence of two characteristic waist-like constrictions, flanking a central extended region. The flagellar P-ring protein FlgA and type IV pilus assembly protein CpaB are two examples of β-clip-domain-containing proteins.
A behavioural response by bacteria whereby a bacterial cell senses a chemical gradient and moves towards or away from the chemical stimulus.
Also referred to as typology. The idea that a specific kind of entity can be defined by an invariant essence. A triangle illustrates essentialism: all triangles have the same fundamental characteristics and are sharply delimited against quadrangles or any other geometric figures. An intermediate between a triangle and a quadrangle is inconceivable. Typological thinking is however unable to accommodate the profligate variation that occurs in biology.
- Establishment clause
A clause from the First Amendment to the American Constitution that states that: 'Congress shall make no law respecting an establishment of religion'. This is now interpreted to forbid any state funding of religious education in the United States.
- Intelligent design
(ID). The concept that some aspects of the natural universe are better explained by an intelligent cause rather than by an undirected process such as natural selection.
- Irreducible complexity
The notion that some biological systems are so complex that they could not function if they were any simpler, and so could not have been formed by successive additions to a precursor system with the same functionality.
- Occam's razor
The principle that the explanation of any phenomenon should make as few assumptions as possible.
- Proton-motive force
Storage of energy as a combination of a proton and voltage gradient across the bacterial inner membrane. The proton-motive force is exploited by the membrane-associated F-type ATPase to generate ATP, and by the flagellar motor to generate torque. In some bacteria, an analogous sodium-motive force drives flagellar rotation.
- SpoA domain
A β-sheet domain found at the C terminus of flagellar proteins FliM and FliN and non-flagellar T3SS proteins such as YscQ and HrcQb.
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Pallen, M., Matzke, N. From The Origin of Species to the origin of bacterial flagella. Nat Rev Microbiol 4, 784–790 (2006). https://doi.org/10.1038/nrmicro1493
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